Abstract. Forest canopies are primary emission sources of biogenic volatile organic
compounds (BVOCs) and have the potential to significantly influence the
formation and distribution of secondary organic aerosol (SOA) mass.
Biogenically-derived SOA formed as a result of emissions from the widespread
forests across the globe may affect air quality in populated areas, degrade
atmospheric visibility, and affect climate through direct and indirect
forcings. In an effort to better understand the formation of SOA mass from
forest emissions, a 1-D column model of the multiphase physical and chemical
processes occurring within and just above a vegetative canopy is being
developed. An initial, gas-phase-only version of this model, the Atmospheric
Chemistry and Canopy Exchange Simulation System (ACCESS), includes processes
accounting for the emission of BVOCs from the canopy, turbulent vertical
transport within and above the canopy and throughout the height of the
planetary boundary layer (PBL), near-explicit representation of chemical
transformations, mixing with the background atmosphere and bi-directional
exchange between the atmosphere and canopy and the atmosphere and forest
floor. The model formulation of ACCESS is described in detail and results
are presented for an initial application of the modeling system to Walker
Branch Watershed, an isoprene-emission-dominated forest canopy in the
southeastern United States which has been the focal point for previous
chemical and micrometeorological studies. Model results of isoprene profiles
and fluxes are found to be consistent with previous measurements made at the
simulated site and with other measurements made in and above mixed deciduous
forests in the southeastern United States. Sensitivity experiments are
presented which explore how canopy concentrations and fluxes of gas-phase
precursors of SOA are affected by background anthropogenic nitrogen oxides
(NOx). Results from these experiments suggest that the level of ambient
NOx influences the pathways by which SOA is formed by affecting the
relative magnitudes and fluxes of isoprene oxidation products emitted from
the canopy. Future versions of the ACCESS model are planned to be
multiphase, including gas- and aerosol-phase chemical and physical
processes, to more fully explore these preliminary results.